Phenylalanine ammonia lyase (PAL; EC 4.3.1.5.) is an enzyme that is found in several plants, yeast, and Streptomyces. PAL catalyzes the nonoxidative deamination of L-phenylalanine to trans-cinnamic acid. The enzyme has a potential role in the treatment and diagnosis of phenylketonuria (Ambrus et al., Science, 201, 837-839 (1978)) and cancer, and is commercially useful for the manufacture of L-phenylalanine from ammonia and t-cinnamate.
Many references describe PAL-producing yeast strains that are useful in fermentation cultures for producing phenylalanine. Rhodotorula glutinis can be employed to obtain PAL activity in the presence of inducer, but the activity reaches a maximum after about six hours of induction and then diminishes thereafter. PAL similarly is rapidly degraded in the absence of the inducer during fermentation and has a half-life of approximately 2-5 hours during fermentations of most Rhodotorula rubra strains.
U.S. Pat. No. 4,598,047 describes mutant strains of Rhodotorula rubra (GX 5902, GX 5903, GX 5904 specifically) that are useful for PAL production. Rhodotorula graminis wild-type strain KGX 39 (also known as GX 5007) is a soil isolate that similarly has PAL activity (Durham et al., J. Bact., 160, 771-777 (1984)). KGX 39 has several advantages over other production strains of Rhodotorula rubra. It grows 15-20% faster and requires less yeast extract, has no L-methionine requirement during induction, and its PAL half-life during fermentation is about 8 to 9 hours. R. graminis KGX 39, however, is undesirable as a production strain due to low PAL titers obtained during fermentation.
An over-producing PAL mutant also has been obtained by mutagenesis of strain KGX 39, as described in U.S. Pat. No. 4,757,015. This mutagenized strain (deposited as ATCC 20804) has high PAL specific activity and titer, high PAL specific productivity, high stability, and lower fermentation times to maximum PAL concentration than any of the previously-available PAL-producing yeast strains.
The use of yeast-derived PAL to produce a variety of optically-active unnatural amino acids having phenylalanine-like structures as chiral synthons for synthesis recently has been described (see, U.S. Pat. No. 5,981,239, incorporated by reference in its entirety herein). According to this reference, the stereospecific introduction of ammonia is accomplished with use of microorganism cells (i.e., cells of the yeast strain Rhodotorula graminis ATCC 20804) as the biocatalyst for the stereospecific conversion. Phenylalanine ammonia lyase from R. graminis ATCC 20804 was found to demonstrate broad substrate specificity for introduction of a molecule of ammonia stereoselectively onto the double bond of a 3-substituted acrylic acid. This newly discovered activity of R. graminis PAL should prove useful commercially.
In particular, phenylalanine and its derivatives also have been used as essential building blocks in the construction of various types of biologically active molecules. For instance, protease inhibitors employed in the treatment of human immunodeficiency virus and human cytomegalovirus infections contain a phenylalanine-like architecture as their pharmacophores. Presently there is a need for a general process of preparing a variety of optically active unnatural amino acids (i.e., amino acids that are not found in nature) having phenylalanine-like structures as chiral synthons for synthesis of these drug candidates. Based on the broad substrate specificity of R. graminis, it would be useful to obtain the polypeptide and nucleic acid sequences of its PAL, e.g., amongst other things, for optimization of its enzymatic activities in these synthesis reactions.
Accordingly, while polynucleotides encoding phenylalanine ammonia lyase have been isolated from the yeasts Rhodosporidium toruloides (PCT WO 88/02824) and Rhodotorula rubra (Filpula et al., Nucleic Acids Research, 16, 11381 (1988), it would be useful to obtain the polynucleotide sequence of still other species. There is a need for strains that can be employed for the production of phenylalanine, phenylalanine analogs, and other optically active unnatural amino acids having phenylalanine-like structures. The present invention thus is directed, amongst other things, to methods, vectors, sequences, and compositions to meet that need. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. The description and examples are provided to enhance the understanding of the invention, but are not intended to in any way limit the scope of the invention.